Most decent DAWs nowadays allow you to change the pan law one way or another.
I also use Reaper, and I always use 0db pan law.
Part of the reason is this thing the OP said: "From what I understood when using the 3db pan law - from center to hard left or right, there is a 3db gain. So the faders need to be adjusted according to the pan again right?"
Theoretically, if the pan law is right for your room, you won't have to adjust gain after panning, that's kind of the whole point. OTOH, whatever pan law you use, if you pan it to one side and it's suddenly not loud enough or too loud, you're going to adjust the volume instinctively anyway. Some people like that the pan knob does some of that for them, but I'm fine with just adjusting by ear.
More important to me is that pan laws stack. If you have a source that peaks at 0db on a track with -3db pan law, and leave the pan pot at center, the output from that track will peak at -3db. If you then route that through a "bus" track with the same pan law and leave that centered, it will come out peaking at -6. I personally just can't have that.
For the most part, the only time pan law is even close to important is if you've got things flying around via automation of the actual pan knob*. I very rarely do that, and luckily if I really feel it's necessary, Reaper allows pan law override on individual tracks.
*There's a video by one of the more popular Reaper tutorial guys where he tries to demonstrate pan law by adding an autopan VST to a track. It completely does not actually do what he's trying to show. If he was automating the pan knob itself, his description would be right on, but since the pan pot itself never moves, it's not really happening. I've asked him to fix it, but I don't know if he's gotten around to it.
